Transmission line type noise filter with reduced heat generation even when large DC current flows therein

ABSTRACT

A transmission line type noise filter connectable between a direct current power supply ( 70 ) and an electrical load component ( 80 ) to pass a coming DC current while attenuating a coming AC current comprises a first conductor ( 11 ), a dielectric layer ( 30 ), a second conductor ( 20 ) as a cathode, a first anode ( 12 ), and a second anode ( 13 ). The first and the second conductors ( 11, 20 ) and the dielectric layer ( 30 ) serve as a capacitance forming portion ( 50 ). The thickness (t) of the first conductor ( 11 ) is selected to substantially restrict temperature elevation of the first conductor ( 11 ), which is caused by DC direct current flowing in the first conductor ( 11 ).

[0001] This invention claims priority to prior Japanese patentapplication JP 2002-222925, the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a noise filter that is mountedin an electronic device or electronic equipment for removing noisegenerated in the device or equipment.

[0003] Digital technologies are important technologies supporting IT(Information Technology) industries. Recently, digital circuittechnologies such as LSI (Large Scale Integration) have been used in notonly computers and communication-related devices, but also householdelectric appliances and vehicle equipment.

[0004] However, high-frequency noise currents generated in LSI chips orthe like are spread from the LSI chips over wide ranges within circuitboards mounting the LSI chips by electric transmission includinginductive coupling with signal wiring or ground wiring on the circuitboards, and further radiated as electromagnetic waves from the signalcables or the like around the circuit boards.

[0005] In a circuit comprising an analog circuit portion and a digitalcircuit portion, electromagnetic interference from the digital circuitportion to the analog circuit portion has been becoming a seriousproblem.

[0006] As a countermeasure therefor, a technique of power supplydecoupling is effective wherein an LSI chip as a source of generation ofhigh-frequency current is separated from a DC power supply system interms of high frequencies. Noise filters such as bypass capacitors havebeen used hitherto as decoupling elements. The operation principle ofthe power supply decoupling is simple and clear.

[0007] A capacitor conventionally used as a noise filter in an ACcircuit forms a two-terminal lumped constant noise filter. A solidelectrolytic capacitor, an electric double-layer capacitor, a ceramiccapacitor or the like is often used therefor.

[0008] When carrying out removal of electrical noise in an AC circuitover a wide frequency band, inasmuch as a frequency band that can bedealt with by one capacitor is relatively narrow, different kinds ofcapacitors, for example, an aluminum electrolytic capacitor, a tantalumcapacitor and a ceramic capacitor having different self-resonancefrequencies, are provided in the AC circuit.

[0009] Conventionally, however, it has been bothersome to select anddesign a plurality of noise filters that are used for removingelectrical noise of a wide frequency band. In addition, there has been aproblem that the use of different kinds of noise filters makes thecircuit high in cost, large in size, and heavy in weight.

[0010] Further, for dealing with higher-speed and higher-frequencydigital circuits, noise filters are desired that can ensure decouplingover a high frequency band and exhibit low impedances even in the highfrequency band.

[0011] However, the two-terminal lumped constant noise filters havedifficulty in maintaining low impedances up to the high frequency banddue to self-resonance phenomena of capacitors, and thus are inferior inperformance of removing high-frequency band noise.

[0012] Therefore, a noise filter is requested that is excellent in noiseremoving characteristic over a wide band including a high frequency bandand that has a small size and a simple structure.

[0013] In order to respond to the request mentioned above, attention isgiven to a transmission line type noise filter, which is connectablebetween a power supply and an electrical load component such as the LSIchip and can pass coming DC current while attenuating coming AC current.

[0014] However, because the DC current to be supplied to the electricalload component passes in the transmission line type noise filter, heatis generated in the transmission line type noise filter. Therefore, thetransmission line type noise filter is serious in heat generation foruse in an electrical circuit having a large DC current flowing therein,and the life of the transmission line type noise filter is thereforeshortened.

SUMMARY OF THE INVENTION

[0015] It is therefore an object of the present invention to provide atransmission line type noise filter with reduced heat generation evenwhen a large DC current flows therein.

[0016] It is therefore another object of the present invention toprovide the transmission line type noise filter that is excellent innoise removing characteristic over a wide band including a highfrequency band and that has a small size and a simple structure.

[0017] A transmission line type noise filter according to the presentinvention is connectable between a direct current (DC) power supply (70)and an electrical load component (80) and can pass a coming DC currentwhile attenuating a coming AC current. The transmission line type noisefilter comprises a first conductor (11) formed in a plate and having alength (L) along a first direction (X) parallel to a transmission line,a width (W) along a second direction (Y) perpendicular to the firstdirection (X), and a thickness (t) along a third direction (Z)perpendicular to the first and the second directions (X, Y), adielectric layer (30) formed on the first conductor (11), a secondconductor (20) formed on the dielectric layer (30), a first anode (12)connected to one end portion of the first conductor (11) in the firstdirection (X) for connecting the first conductor (11) to the directcurrent power supply (70), and a second anode (13) connected to theother end portion of the first conductor (11) in the first direction (X)for connecting the first conductor (11) to the electrical load component(80). The second conductor (20) serves as a cathode connectable to astandard potential. The first and the second conductors (11, 20) and thedielectric layer (30) serve as a capacitance forming portion (50). Thethickness (t) of the first conductor (11) is selected to substantiallyrestrict temperature elevation in the first conductor (11) caused by aDC current flowing in the first conductor (11).

[0018] The first conductor (11) may be made essentially ofvalve-operational metal and an oxidized film of the valve-operationalmetal can make the dielectric layer (30).

[0019] In an embodiment, the valve-operational metal is aluminum, andthe thickness (t) of the first conductor (11) is selected not more than2.0 mm.

[0020] In another embodiment, valve-operational metal is tantalum andthe thickness (t) of the first conductor (11) is selected not more than1.5 mm.

[0021] In another embodiment, the valve-operational metal is niobium andthe thickness (t) of the first conductor (11) is selected not more than1.0 mm.

[0022] In a preferred embodiment, the first conductor (11) and the firstand the second anode (12, 13) are integrally formed in a form of a metalsheet.

[0023] Other objects, features, and advantages of the present inventionwill become apparent from reading the following detailed description ofthis specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIGS. 1A, 1B, and 1C are diagrams showing an exemplary structureof a transmission line type noise filter according to a preferredembodiment of the present invention, wherein FIG. 1A is a plan view,FIG. 1B is a sectional view taken along a line 1B-1B in FIG. 1A, andFIG. 1C is another sectional view taken along a line 1C-1C in FIG. 1A;

[0025]FIG. 2 is a schematic perspective view of a first conductor in thetransmission line type noise filter according to the present invention,for use in describing relationships between the size and heat generationof the first conductor;

[0026]FIG. 3 is a graph showing results from a test for investigating arelationship between the temperature elevation and the thickness of thefirst conductor per different material used in the transmission linetype noise filter according to the present invention;

[0027]FIG. 4 is another graph showing results from another test forinvestigating a relationship among the temperature elevation, thethickness and the length of the first conductor used in the transmissionline type noise filter according to the present invention;

[0028]FIG. 5 is still another graph showing results from still anothertest for investigating a relationship among the temperature elevation,the thickness and the width of the first conductor used in thetransmission line type noise filter according to the present invention;and

[0029]FIG. 6 is a further graph showing results from a further test forinvestigating a relationship among the temperature elevation and thethickness of the first conductor used in the transmission line typenoise filter according to the present invention, and the DC currentapplied to the first conductor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Now, transmission line type noise filters according to preferredembodiments of the present invention will be described hereinbelow withreference to the drawings.

[0031] Referring to FIGS. 1A to 1C, a transmission line type noisefilter according to an embodiment of the present invention isconnectable between a direct current power supply (DC power supply) 70and an LSI chip 80 as an electrical load component and can pass a comingdirect current while can attenuate a coming alternating current.

[0032] The transmission line type noise filter comprises a firstconductor 11, a dielectric layer 30, a second conductor 20, a firstanode 12, and a second anode 13.

[0033] The first conductor 11 is plate-shaped and has a length L along afirst direction X parallel to a transmission line, a width W along asecond direction Y perpendicular to the first direction X, and athickness t along a third direction Z perpendicular to the first and thesecond directions X, Y. The dielectric layer 30 is formed as a film onand around the first conductor 11 in the manner such that opposite endsof the first conductor 11 in the first direction X are exposed. Thesecond conductor 20 is also formed as a film layer on and around thedielectric layer 30. The first anode 12 is connected to one end portionof the first conductor 11 in the first direction X. The first anode 12is for connecting the first conductor 11 to the DC power supply 70. Thesecond anode 13 is connected to the other end portion of the firstconductor 11 in the first direction X. The second anode 13 is forconnecting the first anode 11 to the LSI chip 80. Furthermore, thesecond conductor 20 serves as a cathode connectable to a ground line asa standard potential.

[0034] For example, the first conductor 11 used in the transmission linetype noise filter as a product has the length L of 7.3 or 15.0 mm andthe width W of 4.3 or 11.0 mm.

[0035] The first and the second conductors 11, 20 and the dielectriclayer 30 serve as a capacitance forming portion 50.

[0036] The first conductor 11 and the first and the second anode 12, 13may be integrally formed of an etched aluminum foil 10 in a form of ametal sheet.

[0037] The first anode 12, the second anode 13, and the second conductor20 as the cathode are mounted and electrically connected on first,second and third lands 41, 42, and 43 formed on a circuit board 90 bysoldering, respectively. The first and the second lands 41 and 42 areconnected to a power output terminal of the DC power supply 70 and apower input terminal of the LSI chip, respectively. The third land 43 isconnected to the ground line (not shown), which is the standardpotential common to the DC power supply 70 and the LSI chip 80.

[0038] The transmission line type noise filter can be structured as anelectric chip by covering the filter (packaging) with resin exceptelectrical connecting portions or terminals (not shown) of the firstanode 12, the second anode 13, and the second conductor 20.

[0039] Aluminum (Al), which is a material of the etched aluminum foil10, is a kind of valve-operational metal. In the present invention, thevalve-operational metal represents such a metal that, when oxidized,forms an oxide film, which performs a valve operation. Accordingly, thedielectric 30 can be formed by an oxidized aluminum film of the etchedaluminum foil 10 as the first conductor 11. Although the thickness ofthe dielectric 30 is, for example, 1 μm, it is shown in FIGS. 1B and 1Cwith a thickness more than the actual thickness so as to help in orderto facilitate understanding the structural relationship among componentsof the filter according to the present invention. On the other hand, thesecond conductor 20 comprises a solid electrolyte layer, a graphitelayer, and a silver coating layer formed on the dielectric layer 30 inthis order. Although the thickness of the second conductor 20 is, forexample, 50 μm. The second conductor 20 is also shown in FIGS. 1B and 1Cwith a thickness more than the actual thickness.

[0040] The reason why the aluminum foil is etched is to make the surfaceof the aluminum foil rough and thus to increase the surface area of thedielectric oxide film formed on the foil, which leads to achievement ofa high capacitance.

[0041] In the present invention, the valve-operational metal is notlimited to aluminum, but tantalum (Ta) or niobium (Nb) can also be used.In use of Ta or Nb, it is preferable that the first conductor 11 isformed by sintering powder or a green sheet of tantalum or niobium invacuum atmosphere. Tantalum or niobium sintered body has a roughsurface, and thus the surface area thereof is relatively large.Therefore, the area of an oxidized film, as the dielectric 30, formed ona surface of the sintered body is also relatively large. Thus, thetransmission line type noise filter can be obtained with a highcapacitance.

[0042] The thickness t of the first conductor 11 should be selected tosubstantially restrict the temperature elevation of the first conductor11 caused due to heat generation when a DC current flows in the firstconductor 11. Now, this will be hereinbelow described in detail.

[0043] The transmission line type noise filter, which is connectedbetween the DC power supply 70 and the LSI chip 80 through the circuitboard 90, passes a coming DC current while attenuates a coming ACcurrent. Namely, the DC current supplied to the LSI chip 80 flows in theetched aluminum foil 10 in the form of a metal sheet.

[0044] The DC current is input in the first land 41, passes through thefirst anode 12, the first conductor 11, and the second anode 13, and isthus output from the second land 42. In this case, Joule heat isgenerated in the etched aluminum foil 10, particularly in the firstconductor 11. The temperature of the transmission line type noise filteris therefore increased. The temperature elevation of the transmissionline type noise filter causes a matter such that the life of thetransmission line type noise filter is shortened.

[0045] The temperature elevation of the first conductor 11 by the DCcurrent and its solution by the present invention will be hereinbelowdescribed in detail.

[0046]FIG. 2 is a schematic perspective view of the first conductor 11.The first conductor 11 has the length L, the width W, and the thicknesst. The DC current flows in the first direction X as apparent from FIG.2.

[0047] An amount of heat generated in the first conductor 11 isproportional to the resistance of the first conductor 11. When the firstconductor 11 is constant in its shape and size in a plan view, theelectrical resistance of the first conductor 11 is inverse proportionalto the thickness t of the first conductor. Therefore, the firstconductor 11 is increased in its thickness, the heating value generatedin the first conductor 11 is decreased. On the other hand, the increasedthickness t of the first conductor 11 decreases heat radiation from thefirst conductor 11. The present inventors have found out an appropriateor adaptable range of the thickness t to balance the heat valuegenerated in the first conductor 11 with the heat value radiated fromthe first conductor 11. In more concrete, the adaptable range of thethickness t of the first conductor 11 was determined by the followinginvestigation.

[0048]FIG. 3 shows the test results of regarding the temperatureelevation of several samples for the first conductor 11. In the test,different samples of the first conductor 11 were made from an etchedaluminum foil of the aluminum purity of 99.96%. The different sampleshave the same length L of 1 cm, the same width W of 1 cm, and differentthickness of 0.01 to 5.0 mm. In order to investigate the relationsbetween the thickness t and the temperature elevation, the DC current of30A was continuously applied to flow through each of the samples for 60seconds, which is sufficient for the temperature of each sample to besettled. The test results are shown in FIG. 3. It is noted from FIG. 3that the thickness t of the first conductor 11 made essentially ofaluminum should be selected 2.0 mm or less so as to substantiallyrestrict the temperature elevation.

[0049] Furthermore, regarding other samples of the first conductor 11made essentially of sintered tantalum and sintered niobium,respectively, the similar investigation was carried out. The testresults are also shown in FIG. 3.

[0050] Consequently, it is noted from FIG. 3 that the thickness t of thefirst conductor 11 made essentially of tantalum should preferably beselected 1.5 mm or more so as to substantially restrict the temperatureelevation. Further, the thickness t of the first conductor 11 madeessentially of niobium should preferably be selected 1.0 mm or more.

[0051]FIG. 4 shows results from another test for investigating anyeffect of the length L of the first conductor 11 to the relationshipbetween the temperature elevation and the thickness t of the firstconductor 11. In the test, different samples were made from an etchedaluminum foil of the aluminum purity of 99.96%. The different sampleshave different lengths L of 0.5, 1.0, 2.0, and 4.0 cm, the same width Wof 1 cm, and different thickness of 0.01 to 5.0 mm. The DC current of30A was continuously applied to flow through each of the samples for 60seconds, which is sufficient for the temperature of each sample to besettled. The test results are shown in FIG. 4. It is noted from FIG. 4that the length L of the first conductor 11 does not almost affect tothe relationship between the temperature elevation and the thickness t,and that the thickness t of the first conductor 11 made essentially ofaluminum should be selected 2.0 mm or less so as to substantiallyrestrict the temperature elevation.

[0052]FIG. 5 shows results from still another test for investigating anyeffect of the width W of the first conductor 11 to the relationshipbetween the temperature elevation and the thickness t of the firstconductor 11. In the test, different samples were made from an etchedaluminum foil of the aluminum purity of 99.96%. The different sampleshave the same length L of 1 cm, different widths W of 0.2, 0.5, 1.0, and1.5 cm, and different thickness of 0.01 to 5.0 mm. The DC current of 30Awas also continuously applied to flow through each of the samples for 60seconds, which is sufficient for the temperature of each sample to besettled. The test results are shown in FIG. 5. It is noted from FIG. 5that although difference of the width W of the first conductor 11affects to the temperature elevation in a region of thickness t morethan 2.0 mm, the thickness t of the first conductor 11 should beselected 2.0 mm or less so as to substantially restrict the temperatureelevation.

[0053]FIG. 6 shows a further test results investigating affect of the DCcurrent applied to the first conductor 11. In the test, differentsamples were also made from an etched aluminum foil of the aluminumpurity of 99.96%. The different samples have the same length L of 1 cm,the same width W of 1 cm, and different thickness of 0.01 to 5.0 mm.Each of different DC currents of 5A, 10A, and 30A was continuouslyapplied to flow through each of the samples for 60 seconds. The testresults are shown in FIG. 6. It is noted from FIG. 6 that although thevalue of the DC current affects to the temperature elevation in a regionof thickness t more than 2 mm, the thickness t of the first conductor 11made essentially of aluminum should be selected 2.0 mm or less so as tosubstantially restrict the temperature elevation.

[0054] It is preferable that the thickness t of the first conductor 11made of a material such as aluminum, tantalum, or niobium is not lessthan several μm, in order to secure the mechanical strength of the firstconductor 11 and so on.

[0055] While the present invention has thus far been described inconjunction with several embodiments thereof, it will readily bepossible for those skilled in the art to put the present invention intopractice in various other manners.

[0056] For example, the noise filter according to the present inventioncan be connected to the LSI and be packaged with the LSI in a commonpackage made of resin so that an LSI chip having a noise filter isstructured.

What is claimed is:
 1. A transmission line type noise filter connectablebetween a direct current power supply (70) and an electrical loadcomponent (80) for passing a coming DC current while attenuating acoming AC current, said transmission line type noise filter comprising:a first conductor (11) formed in a plate shape and having a length (L)along a first direction (X) parallel to a transmission line, a width (W)along a second direction (Y) perpendicular to said first direction (X),and a thickness (t) along a third direction (Z) perpendicular to saidfirst and said second directions (X, Y); a dielectric layer (30) formedon said first conductor (11); a second conductor (20) formed on saiddielectric (30); a first anode (12) connected to one end portion of saidfirst conductor (11) in said first direction (X) for connecting saidfirst conductor (11) to the direct current power supply (70); and asecond anode (13) connected to the other end portion of said firstconductor (11) in said first direction (X) for connecting said firstconductor (11) to the electrical load component (80); said secondconductor (20) serving as a cathode connectable to a standard potential;said first and said second conductors (11, 20) and said dielectric layer(30) providing a capacitance forming portion (50); and said thickness(t) of said first conductor (11) being selected to substantiallyrestrict temperature elevation of said first conductor (11) caused by DCcurrent flowing in said first conductor (11).
 2. The transmission linetype noise filter according to claim 1, wherein said first conductor(11) is made essentially of valve-operational metal and said dielectric(30) is made of an oxidized film of said valve-operational metal.
 3. Thetransmission line type noise filter according to claim 2, wherein saidvalve-operational metal is aluminum, and wherein said thickness (t) ofsaid first conductor (11) is 2.0 mm or less.
 4. The transmission linetype noise filter according to claim 2, wherein said valve-operationalmetal is tantalum, and wherein said thickness (t) of said firstconductor (11) is 1.5 mm or less.
 5. The transmission line type noisefilter according to claim 2, wherein said valve-operational metal isniobium, and wherein said thickness (t) of said first conductor (11) 1.0mm or less.
 6. The transmission line type noise filter according toclaim 1, wherein said first conductor (11) and said first and saidsecond anode (12, 13) are integrally formed in a form of a metal sheet.